The field of patent application Ser. No. 09/230,339, commonly owned, is that of silicone compositions which are crosslinkable by activation, which can be used in particular to form an anti-adhesive coating or film for a fibrous or non-fibrous support, for example a support made of paper or the like, or alternatively made of natural or synthetic polymer.
More specifically, the invention relates, in the first of these aspects, to a process for the continuous coating of a travelling support strip (e.g. paper, fabric, polymer), in order to give it anti-adhesion properties, the said coating being carried out using a silicone composition based on polyorganosiloxane (POS).
The anti-adhesive POSs which are of interest in the context of the invention taken in its first aspect are of the type which are crosslinkable rapidly (xe2x80x9cfast curexe2x80x9d) and at low temperature (low-temperature cure), by hydrosilylation or by dehydrogenopolycondensation, thermally or by a combination of heat and radiation (UV radiation or an electron beam). Such low-temperature cure and fast cure silicone compositions will be defined by the name: LTC/FC silicone systems.
The invention also relates to the device involved in the continuous coating process.
The anti-adhesive silicone compositions concerned comprise at least one POS A, a crosslinking agent B which is a POS and a catalyst. These ternary compositions can only exist transiently in non-crosslinked form. The reason for this is that, irrespective of the crosslinking mechanism concerned: hydrosilylation or dehydrogenopolycondensation, POSs of SiH type placed in the presence, in the first case, of POSs of SiVi type and of platinum hydrosilylation catalyst, or placed in the presence, in the second case, of POSs of SiOH or SiOR type and of platinum or tin condensation catalyst crosslink relatively quickly. The kinetics of crosslinking depends in particular on the reaction temperature.
These silicone systems are also characterized by bath lifetimes BLTs. The term BLT denotes the time required to double the dynamic viscosity measured at 30xc2x0 C. The dynamic viscosity can be measured using a Brookfield viscometer according to the indications of AFNOR standard NFT 76102 of May 1982.
For the anti-adhesion treatments of supports (paper, fabric or polymer film), which consist in coating the surface of these supports with the abovementioned silicone compositions, it is imperative to apply and spread the said compositions when they are in non-crosslinked liquid form and thus entirely suited for use in the said operations.
Once the supports are coated with silicone composition, they are subjected to heating so as to accelerate their crosslinking.
The problematics underlying the invention, taken in its first aspect, described and claimed in the patent application of Ser. No. 09/230,339, commonly owned, now U.S. Pat. No. 6,387,452 is the provision of an anti-adhesive silicone coating process which:
1) offers a compromise between the parameters of reactivity; crosslinking temperature; crosslinking kinetics; and bath lifetime; and
2) involves a procedure for accurate metering and the preparation of a homogeneous mixture of the ingredients of the silicone composition, by using suitable means and by opting to vary operating parameters which are carefully selected.
With respect to point 1): as regards the crosslinking temperature, it is desirable to be able to allow low temperatures (which can be less than or equal to 110xc2x0 C.), thus allowing the coating to be spread and crosslinked on heat-sensitive supports such as, for example, polyethylene, polypropylene, PVC and (to a lesser extent) polyethylene glycol terephthalate; as regards the crosslinking kinetics, it is also desirable to have available a process in which the silicone composition may be crosslinked, even at low temperature, according to high crosslinking kinetics so as to achieve gains in production efficiency and viability; and, as regards the BLT parameter, it is also desirable to have available a process which allows a reduction in the crosslinking temperature, while at the same time retaining bath lifetimes, for the non-crosslinked liquid silicone composition, which are of an acceptable level to allow industrial users a sufficient margin for carrying out the silicone coating at the required industrial rate and scale.
With respect to point 2), the carefully selected operating parameters are:
the bath lifetime, which is controlled by varying the nature and/or proportions of the components A, B and C and/or of their possible premixes,
the homogeneity of the mixture, which is controlled via the DSC signature, allowing calibration relative to a reference mixture, and
the flow rate of production of homogeneous mixture, which is adjusted to the rate of consumption with or without formation of a coating buffer tank, the mixture thus metered and homogenized being conveyed directly to the coating head of the machine, such that only the amount of mixture required to feed the coating head is prepared.
Thus, the invention according to the patent application of Ser. No. 09/230,339, commonly owned, taken in its first aspect, relates more specifically to a process for the continuous coating of a travelling support strip in order to give it anti-adhesion properties, the said coating being carried out using a silicone composition comprising: at least one polyorganosiloxane A, at least one crosslinking agent B which can be crosslinked by hydrosilylation and/or by dehydrogenopolycondensation, and an effective amount of catalyst C,
characterized in that it consists, essentially:
1. in continuously mixing in measured amounts, in particular, the POS(s) A bearing reactive groups RGa, the crosslinking agent(s) B bearing reactive groups RGb, and the catalyst C; by:
optionally preparing at least one premix AB and/or AC and/or BC when this premix can be prepared,
selecting mixing conditions such that:
the bath lifetime (BLT) is less than 10 h,
the bath homogeneity is such that its differential scanning calorimetry DSC signature comprises at least one essentially Gaussian peak showing,
a peak start temperature T1,
a peak summit temperature T2,
a peak end temperature T3,
this peak being characterized by:
-i- T3xe2x89xa6110xc2x0 C.,
-ii- xcex94T=T3xe2x88x92T1xe2x89xa630xc2x0 C., T3 possibly not being
able to satisfy the parameter -i- above, and
-iii- xcex94T=xcex94T refxc2x110xc2x0 C., xcex94T ref corresponding to T3ref-T1ref, obtained from the DSC signature peak of a reference ABC mixture prepared according to the procedure Pr defined below in the present specification,
the flow rate of production of the homogeneous mixture is between a value corresponding to the rate of consumption of the mixture on the coating site(s) and a value which determines the constitution of a buffer tank, this flow rate value being such that the duration separating the time at which the components A, B and C are placed together and the time of application of the composition ABC to the travelling support strip is less than or equal to the BLT,
2. in conveying, simultaneously or otherwise, the composition directly from the mixer to the coating site(s),
3. in coating the travelling support strip with the composition,
4. in allowing the crosslinking to take place.
The DSC (Differential Scanning Calorimetry) analysis is carried out, in a manner which is known per se, using a Metler TA 4000 type machine with the following operating parameters: rate of temperature increase: 10.0xc2x0 C./min; mass of the test sample: 15 mg; working in an open aluminium crucible.
The inventors have, to their credit, developed a continuous coating method which makes it possible industrially to use thermal silicone systems which crosslink (polyaddition or dehydrogeno-polycondensation) at low temperatures and with BLTs as short as values of less than 10 hours.
This innovation requires both expertise in silicone chemistry and, on the one hand, mastery of techniques for precise metering in ratios which can be as low as 1:100 or less than 1:100, and, on the other hand, for continuous intimate mixing of more or less viscous products which can be used at low flow rate. Given the precision of the meterings, which can be of the order of 1%, this results in a considerable improvement in the quality of the silicone coating, as well as excellent consistency of the production of quality finished products by the users.
Advantageously, the procedure Pr for establishing the reference mixture ABC in order to arrive at the comparative parameter xcex94Tref for the BLT peaks of the mixture consists in mixing the same constituents A, B and C in the same proportions in order to prepare, in a container, a mixture of 250 cm3, with moderate stirring using an impeller rotating at 1000 rpm for at least 15 min, at room temperature (25xc2x0 C.).
In practice, if need be, the buffer tank can be established on the member for coating the travelling support strip. This tank is, for example, between 10 and 30 min of consumption of the composition used to carry out the coating.
It is also stated in the patent application of Ser. No. 09/230,339, commonly owned, now U.S. Pat. No. 6,387,452 that, when the homogeneity of the mixture ABC is such that it is reflected by a DSC peak for which the parameters (i), (ii) and (iii) are cumulatively satisfied, the invention described and claimed therein then relates to xe2x80x9can LTC/FC silicone system, given that such systems, although preferred in accordance with the invention, do not, however, exclude other silicone systems which crosslink at higher temperature and/or slightly less quicklyxe2x80x9d.
It thus emerges that the invention in question also has a second aspect which consists of a process for the continuous coating of a travelling support strip, carried out using a silicone composition in which the POSs of interest are now, in particular, those which can be crosslinked between themselves at a higher temperature and/or slightly less quickly than the POSs of the LTC/FC silicone systems.
The present invention attempts to satisfy the following objects.
The first of these objects is to provide a continuous coating process carried out using a silicone composition which does not correspond to an LTC/FC silicone system, and which continues to combine, as explained in the patent application of Ser. No. 09/230,339, commonly owned, U.S. Pat. No. 6,387,452 on the one hand, an accurate metering procedure (using metering means, in particular volumetric means, which are capable of supplying a metered amount of each of the constituents of the silicone composition) and, on the other hand, a procedure for the homogeneous mixing of the constituents of the silicone composition (using homogeneous mixing means and premixing means, these means being of dynamic and/or static nature), with the aim of retaining the advantages mentioned above, in particular the considerable improvement in the quality of the silicone coating, as well as excellent consistency of the production of quality finished products.
The first object to be achieved comprises the development of a continuous coating process which has the abovementioned specifications and which can be used advantageously to coat a travelling flexible support strip consisting of papers of various types (supercalendered, laminated, etc.), cardboards, cellulose sheets, metal sheets, plastic (polyester, polyethylene, polypropylene, etc.) films, in order to give it anti-adhesive properties.
The first object to be achieved also comprises the development of a continuous coating process which has the abovementioned specifications and which can be used advantageously to coat a travelling flexible support strip consisting of a textile material such as, for example, fibrous, woven, knitted or non-woven supports, in order to give it properties, in particular, of mechanical protection and/or strength, the said coating being carried out using a silicone polyaddition composition which has the particular feature of belonging to the group of silicone elastomer polyaddition compositions for coating.
The second essential object, which the invention attempts to satisfy, is to provide a device which can be used in particular for carrying out the process of continuous coating of a textile material using a silicone elastomer polyaddition composition.